1. Field of the Invention
The present invention relates to a valve seat for use in an internal combustion engine, and more particularly, to a composite valve seat made of two kinds of sintered alloys.
2. Description of the Prior Art
As illustrated in FIG. 1, a spot surface 31 of a valve seat 3 always strikes a valve 4. Therefore, the spot surface 31 is subject to abrasion. In addition, when the valve seat 3 is positioned at the exhaust gas side of an engine, an inner peripheral surface 32 thereof is always exposed to high temperature exhaust gas. Therefore, the inner peripheral surface 32 is easily subject to corrosion. For valve seats used at the exhaust gas side, special materials having good abrasion resistance, heat resistance, and corrosion resistance are required. In these days, sintered alloys are usually used.
However, the sintered alloys which have good abrasion resistance, heat resistance and corrosion resistance are expensive since they contain large amounts of expensive elements, e.g., cobalt (co), nickel (Ni) and chromium (Cr). In view of the high material costs, composite valve seats as illustrated in FIGS. 2 and 3 are made which have only a zone 2 that is made of a special second sintered alloy. The remaining part of the valve seat constitutes a zone 1 which is made of a first sintered alloy 1 which is an ordinary less expensive sintered alloy. As can be seen by comparing the valve seats illustrated in FIGS. 2 and 3 the volume of the second sintered alloy contained in the valve seat of FIG. 3 is less than the volume of the second sintered alloy contained in the valve seat of FIG. 2. However, the volume of the second sintered alloy contained in the valve seat of FIG. 2 still tends to be excessive which unduly raises production costs.